1. Field of the Invention
This invention relates to a transducer for microwave propagation. More specifically, the invention relates to several transducers electrically connected in series to form an array for an acoustic delay device.
2. Description Of The Prior Art
In the field of microwave technology, bulk mode acoustic delay lines are well known. Bulk acoustic delay lines provide radio frequency memory for periods from submicrosecond to several microseconds. These bulk acoustic delay lines (see FIG. 1) are typically a body 10 formed of a crystal material such as quartz or sapphire. At opposing parallel surfaces 12, 14 of the body piezoelectric transducers 16, 18 are formed. An electrical signal is provided to transducer 16 at one surface 12 of the crystal body 10. Transducer 16 then generates an acoustic compression wave in the crystal body 10 which travels through the body 10. When the compression wave reaches opposing surface 14 of crystal body 10 it causes the second piezoelectric transducer 18 at that end 16 to vibrate mechanically thus generating a corresponding electric signal in transducer 18. Thus the propagation of the acoustic compression wave through a crystal material provides a significantly slower time of propagation than would the radio frequency electrical signal propagating an equivalent distance. A radio frequency electrical signal is thereby converted to a mechanical wave which is then converted back to an electrical signal, providing the delay effect. These delay lines are typically used in microwave applications such as radar and microwave communications. These devices are well known in the art and various modifications are also well known.
The desirable characteristics for a transducer include low conversion loss together with large bandwidth. Typical prior art devices typically include a compromise based on selection of materials and the required conversion loss and bandwidth. Conversion loss is defined as the one way conversion of electric to acoustic power or vice versa. In the typical case when two transducers are assembled at opposite ends of an acoustic substrate to form a delay line, the total loss for the configuration is the sum of the transducer conversion losses plus any loss due to propagation through the acoustic substrate which includes the diffraction loss as well as the material loss.
FIG. 2 shows a typical frequency response of a typical prior art delay line over a range of 2 gigahertz to 6 gigahertz along the horizontal axis with the signal strength shown in dB along the vertical axis. As shown, in the 2 gigahertz to 6 gigahertz range, there is a wide range of frequency response with a strong central peak. This response is undesirable in that this particular device provides a weak signal at the high and low portions of the frequency spectrum.
This is because typically just one transducer element is incorporated at each end of the crystal body. The trade-off has to be made between the conversion loss, which favors a smaller aperture, at the transmitting and receiving transducers, and the diffraction loss in the crystal body which favors a larger aperture Additionally, the problems with these known delay lines are exacerbated where the electrical signals are over a bandwidth rather than at one frequency as the transducer's high Q-factor becomes the dominant factor in conversion loss. In many microwave applications, it is necessary to deal with a range of frequencies. This is especially true in the case of radar warning systems which receive a variety of different frequencies in order to detect various threats.
As a remedy, it is well known to electrically connect several transducers in series. This allows a larger individual transducer aperture while maintaining the same overall impedance. In the prior art the series connection requires the use of air bridges to connect the various transducers. Both the air bridges and transducers are formed by complex semiconductor processing methods. In doing so, the individual cell's diffraction loss is reduced although the conversion loss remains relatively the same. However, one very important aspect neglected here is with the provision of several transducers connected in series, the transducer array produces null spots due to the interference of the different transducers at various distances from the transducer.
The result is a beam pattern that can be completely lost if either a row or a column or both of the arrays have an even number of transducers, which is the structure disclosed in U.S. Pat. No. 3,688,222 issued Aug. 29, 1972, to Lieberman. Moreover, if the array has an odd number of transducers in either a row or column, the signal received by each transducer at the opposite end of the acoustic substrate will not be equal. This causes reradiation phenomenon.
FIG. 3 shows such a 2.times.3 transducer array including transducers T.sub.11, T.sub.12, T.sub.13, T.sub.21, T.sub.22, ..., T.sub.23 arranged on one end of an acoustic substrate and electrically connected in series. Also shown are the corresponding beam patterns of radiated acoustical energy into the acoustic substrate along the indicated axes.
FIG. 4a shows a 3.times.3 transducer array arranged on one end of an acoustic substrate with transducers T.sub.11, T.sub.13, T.sub.21, T.sub.22 T.sub.23, Y.sub.31, T.sub.32, T.sub.33, electrically connected in series as shown. Each transducer T.sub.11, T.sub.12, T.sub.13, T.sub.21, T.sub.22, T.sub.23, T.sub.31, T.sub.32, T.sub.33, has an equivalent circuit as shown in FIG. 4b of one capacitor C11, :., C33 combined with one resistor R.sub.11, R.sub.33. Since the transducers T.sub.11, T.sub.12, T.sub.13, T.sub.21, T.sub.22, T.sub.23, T.sub.31, T.sub.32, T.sub.33 are series connected, the current Imn through any transducer T.sub.mn is equal to the current I.sub.11 through the first transducer T.sub.11. For the central transducer T.sub.22 the total current I.sub.22 =I.sub.22.sup.+ +I.sub.22, .sup.-, where I.sub.mn.sup.+ denotes incident current and I.sub.mn.sup.- denotes reflected current. Thus, I.sub.22.sup.+ &lt;I.sub.mn.sup.30 for 1.ltoreq.m, n.ltoreq.3. Therefore I.sub.22.sup.- .ltoreq.I.sub.mn.sup.-, and this is the value of the reradiated signal from each transducer, as a result of the non-uniform beam pattern. In other words, the surrounding 8 cells partly become the load for the center cell.
This reradiation phenomenon is a major deficiency of prior art m by n transducer arrays, and this deficiency is not believed to have been disclosed previously.
It is also known in the art to provide on either end of a crystal body 22 a transducer array which as shown in plan view in FIG. 5a is a mosaic consisting of six pie-shaped segments 24-1, 24-2, 24-3, 24-4, 24-5, 24-6 of a circle, with each transducer 24-1, 24-2, 24-3, 24-4, 24-5, 24-6 electrically connected in series to the adjacent transducer by connections 26-1, 26-2, 26-3, 26-4 26-5. This pattern has the advantage of acting as a large point source and thus reducing diffraction loss and eliminating the undesirable null spots.
This mosaic structure still has the disadvantages that achievable bandwidth remains the same as in a single dot transducer and that fabrication of this structure requires use of the aforementioned air bridge connections 26-1, ..., 26-5, which are the electrical connections between adjacent segments. FIG. 5b shows a cross section through line A-A of FIG. 5a. Shown in FIG. 5b is acoustic substrate 22, and air bridge 26-1 connecting transducers 24-1 and 24-2, with air gap 27 under air bridge 26-1. Instead of resting on a solid substrate as is typical of semiconductor fabrication technology, the air bridge 26-1 therefore must have a small empty gap 27 of air underneath it. While electrically this is not a problem, fabrication of such air bridge structures is relatively difficult, resulting in low yield and a very expensive device.
Thus a recurring problem in the development of microwave acoustic delay lines, amplifiers, and related devices is the design of transducers that provide good response and are not too expensive, i.e., difficult to fabricate.
Another problem typically encountered with prior art delay lines is called "triple travel suppression." This means that a signal traveling through the acoustic substrate typically bounces between the end surfaces making an extra round trip through the acoustic substrate and so provides a spurious signal due to reflection inside the transducer. Larger aperture, i.e. larger diameter, transducers typically have more triple travel than do smaller aperture transducers. This is because the diffraction loss can be viewed as an acoustic padding or attenuator.